1. Field of the Invention
The present invention relates to an electron wave interference device which is controlled by irradiating a light and a method of controlling an interference current by light by using such a device.
2. Related Background Art
It is considered that a system using light in a light transmission, a light conversion, a light computing device, or the like is a key system to realize a large capacity and a super high speed information processing in the future. Among them, the light communication device needs a super high speed photodetector and the light conversion device needs a photodetector which can be integrated at a high density.
To meet the above requirements, a method of controlling a quantum interference current by the light has been proposed by Yamanishi et al., "The Records of the Society of Applied Physics of Japan", page 27-z-1, Autumn, 1989. The principle of the above method will now be described hereinbelow.
An arm 71 on one side of an interferometer for an electron wave comprising a quantum thin line 70 as shown in FIG. 1, is irradiated by a light 72. In this case, a photon energy of the incident light 72 is set to be sufficiently smaller than an excition energy gap of the quantum thin line 70, thereby virtually causing a carrier, that is, causing a virtual transition. The virtual carrier generated in the arm 71 which has been excited by the light decreases an effective potential for an electron wave which propagates in the arm 71 through an exchange interaction according to an exchange energy, so that a quantum interference between the electron waves which propagate in two arms 71 and 73 can be controlled. At this time, an electron/hole carrier pair which has virtually been excited doesn't contribute to the conduction because such a pair relates to a coherent pair excitation.
Since it is a feature of the above method that structure is simple and quantum wave interference is controlled by virtual excitation by light, a switching time of the device is not limited by a CR time constant and a recombination life.
Another conventional technique will now be described. FIG. 2 is a schematic cross-sectional view showing a structure of a quantum interference device which has been proposed by S. Datta et al., "Applied Physics Lett. 48", page 487, 1986.
According to the above structure, two GaAs quantum well layers 80 and 81 are laminated in a z direction through an AlGaAs barrier layer 82. Since both edges of the barrier layer 82 are thin, a considerable tunneling exists between the well layers 80 and 81. However, since the barrier layer 82 in the central portion is thick, a tunneling hardly exists. The electron wave which enters from a contact 83 at the left edge is first coherent in two quantum wells 80 and 81, so that phases of the electron waves in the quantum wells 80 and 81 are coincident. Now, .vertline.A&gt; denotes an asymmetrical wave function of the electron wave and .vertline.S&gt; denotes a symmetrical wave function of the electron wave. In the central portion, namely, in the portion between x=0 and x=L, a magnetic field is applied in the y direction to the electron wave which has been propagated in the x direction so as to cause a phase difference of only .pi. between the upper and lower channels 80 and 81. This means that a band structure of the well layers 80 and 81 and an intensity of magnetic field which is applied are set so as to cause a phase difference of .pi.. By applying a magnetic field as mentioned above, two electron waves which are interfered at the right edge and whose wave functions are respectively expressed by .vertline.1&gt; and .vertline.2&gt; are reflected because their energies are set to high levels. In other words, when the wave functions of the electron waves are recombined to one wave function at the right edge, a resultant waveform is set into the second state .vertline.A&gt; instead of the base state shown by .vertline.S&gt;. However, since the electron wave has only an energy corresponding to .vertline.S&gt;, it is reflected.
On the other hand, if the magnetic field is controlled such that the phase difference is equal to a value which is integer times as large as 2.pi., one recombined electron wave can pass through the right edge and the same output current as the input current is obtained.
Therefore, a conductance shown in an axis of ordinate in FIG. 3 is expressed by a periodic function of the applied magnetic field, so that a passing current can be controlled in a wide range by modulating the magnetic field.
However, the structure of the foregoing former conventional example has the following problems. Consider the case of forming the quantum thin line 70 by GaAs as a compound semiconductor. To cause a virtual excitation from a valence band to a conduction band, it is necessary to use an irradiation light having a wavelength of 0.8 .mu.m due to the relation with an energy interval. However, since lengths of arms 71 and 73 of the interference device are equal to at most 1 .mu.m, it is very difficult to converge the light to only one side of the two arms 71 and 73 existing in such a micro region due to the relation with a diffraction limit.
The foregoing latter conventional example also has the following problems.
First, since the interference current is controlled by the magnetic field, high speed modulation is difficult. This is because an impedance of the coil becomes too large for a high frequency current which is needed to modulate the magnetic field at such a high speed. A frequency of tens of MHz is an upper limit frequency.
Second, a regrowth is preferable as necessary to form the structure as shown in FIG. 2. First, a structure is grown until the AlGaAs barrier layer 82 exists between two well layers 80 and 81 by a molecular beam epitaxy (MBE) method or an organic metal chemical vapor phase deposition (MO-CVD) method. After that, the structure is taken out into the atmosphere and is etched to form a thick portion and a thin portion of the barrier layer 82.
However, at this time, a surface oxidation and an adsorption of impurities cannot be avoided. Therefore, the GaAs quantum well layer 81 grown on the above structure has a rough hetero interface and contains many impurities and defects. Consequently, the electrons which run in the GaAs quantum well layer 81 are scattered and the phase is disturbed, so that an on/off ratio of the interference current decreases.